Critical tests of variational transition state theory and semiclassical tunneling methods for hydrogen and deuterium atom transfer reactions and use of the semiclassical calculations to interpret the overbarrier and tunneling dynamics

Variational transition state theory with semiclassical tunneling calculations is used to calculate rate constants which are compared to the best available quantum mechanical results for three cases involving H atom transfer and three cases involving D atom transfer. The mass combination is heavy-light-heavy, as appropriate for testing semiclassical methods for proton and deuteron (or hydride) transfer in organic and biochemical systems. In five of the six cases at 312.5 K and five (but not the same five) of the six cases at 423.2 K, semiclassical results based on an uncoupled treatment of the vibrational-rotational modes agree with the the best available quantum results within the reliability of the latter. In the other two cases the discrepancies are larger, a factor of 3.1 at 312.5 K and a factor of 2.3 at 423.2 K, and possible reasons for this are discussed. In particular, the effect of a coupled treatment of the bending degrees of freedom in these two cases is presented and compared to the uncoupled treatment. Coupling the bending modes reduces the maximum discrepancy at 312.5 K to a factor of 2.4 but increases the largest discrepancy at 423.2 K to a factor of 4.5. The approximate semiclassical calculations are used to interpret the dynamics, especially in cases where agreement with the fully quantal results is very good.